As far as I know, Natural link building is same as back linking when other blogger, webmasters and website owners link to your content, images etc. Because they think it's useful for their readers and viewers. For other details you follow this resources: http://www.sourcepep.com/top-16-ways-to-get-natural-links-to-your-website/
natural links can be preferred and some website that has good pr links can be chosen. it is better to keep the outreach short and valid